Rx delay line inteferometer tracking in closed-loop module control for communication

ABSTRACT

The present invention is directed to a communication signal tracking system comprising an optical receiver including one or more delay line interferometers (DLIs) configured to demultiplex incoming optical signals and a transimpedance amplifier configured to convert the incoming optical signals to incoming electrical signals. The communication signal tracking system further includes a control module configured to calculate a bit-error-rate (BER) of the incoming electrical signals before forward-error correction decoding, and use the BER as a parameter for optimizing settings of the one or more DLIs in one or more iterations in a control loop and generating a back-channel data.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S.application Ser. No. 16/408,256, filed May 9, 2019, which is acontinuation of and claiming priority to U.S. application Ser. No.15/633,353, filed Jun. 26, 2017, now issued as U.S. Pat. No. 10,333,627on Jun. 25, 2019, commonly assigned and incorporated by reference hereinfor all purposes.

BACKGROUND OF THE INVENTION

The present invention is directed to communication systems and methods.

Over the last few decades, the use of communication networks exploded.In the early days of the Internet, popular applications were limited toemails, bulletin board, and mostly informational and text-based web pagesurfing, and the amount of data transferred was usually relativelysmall. Today, Internet and mobile applications demand a huge amount ofbandwidth for transferring photo, video, music, and other multimediafiles. For example, a social network like Facebook processes more than500 TB of data daily. To move a large amount of data, opticalcommunication networks are often used.

With high demand for communication networks came high demand for qualitynetworking devices. In high-speed communication systems, havingoptimized optical transceivers can meaningfully improve performance. Forexample, various parameters of optical transmitter, such as biasvoltages for modulator and laser devices, can be adjusted and optimizedin a communication system for improved performance.

Over the past, there have been various techniques for optimizingparameters and settings for optical transceivers. Unfortunately,existing techniques are inadequate for reasons explained below. Improvedmethods and systems for optimizing optical communication devices aredesired.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to communication systems and methods.According to an embodiment, delay line interferometers (DLIs) in areceiver module of an optical transceiver are configured with a controlloop to optimize overall bit-error-rate (BER) of communication signalsagainst any drift. The DLI control is further coordinated with activeBER-based wavelength control in a transmitter module of the opticaltransceiver, both being operated alternatively in time or frequency.There are other embodiments as well.

According to an embodiment, the present invention a communication signaltracking system. The communication signal tracking system includes anoptical receiver comprising one or more delay line interferometers(DLIs) configured to demultiplex incoming optical signals, one or morephotodetectors converting the incoming optical signals to currentsignals, and a transimpedance amplifier configured to convert thecurrent signals to voltage signals. Additionally, the communicationsignal tracking system includes a control module configured to calculatea bit-error-rate (BER) based on the voltage signals, and use the BER asa parameter in one or more iterations for optimizing settings of the oneor more DLIs in a control loop and generating a back-channel data.

According to a specific embodiment, the control module is configured toexecute a search of a restart point in a two-dimensional region definedby the first bias point for the first DLI and the second bias point forthe second DLI, based on a determination that the BER of the incomingelectrical signals does not meet the first threshold. The search of arestart point includes starting from a last-known good point to adjustthe first bias point and the second bias point by a minimum increment ordecrement in a spiral path in a two-dimensional pattern up toend-of-life limits until the BER of the incoming signals meets the firstthreshold.

According to another embodiment, the present invention provides acommunication system with signal tracking. The communication systemincludes a communication link, a first transceiver comprising a firstcontrol module and a first receiver, and a second transceiver comprisinga second control module and a second transmitter. The second transceiveris configured to send an optical signal to the first transceiver and toreceive back-channel data from the first transceiver. The first receiveris configured to receive the optical signal and convert the opticalsignal to an electrical signal. The first control module is configuredto calculate a bit-error-rate (BER) based on the electrical signal andto execute a first iterated operation of optimizing the first receiver.The second transmitter is configured to generate the optical signal andtransmit to the first receiver. The second control module is configuredto execute a second iterated operation of optimizing wavelength of theoptical signal. The second iterated operation is alternate in time withthe first iterated operation controlled by a back-channel datatransmitted from the first transceiver to the second transceiver.

According to yet another embodiment, the present invention provides amethod for tracking an optical signal in receive side of a communicationsystem. The method includes receiving an optical signal by a delay lineinterferometer (DLI) in a near-end receiver of a communication system.Additionally, the method includes calculating a first bit-error-rate(BER) of the optical signal. The method further includes determining theDLI to have a low signal-to-noise ratio based on that the first BERmeets a threshold. Furthermore, the method includes adjusting a setpoint of the DLI in a first iterated operation to result in a second BERof the optical signal converged to a value smaller than the first BER.Moreover, the method includes holding the optimized set point of the DLIand transmitting the second BER in real time through a back-channel fromthe near-end receiver to the far-end transmitter of the communicationsystem to adjust a transmitter setting in a second iterated operation toresult in a third BER converged to a value no greater than the secondBER.

It is to be appreciated that embodiments of the present inventionprovide many advantages over conventional techniques. Among otherthings, by measuring actual signal characteristics by a receivingoptical transceiver of the data communication path, adjustments made bya transmitting optical transceiver improve data transmission qualitybetter than existing techniques, where typically one-time factorysettings are applied to optical transceivers. For example, adjustmentssuch as wavelength control may be specific to the optical link andactual operating conditions (e.g., temperature, interference, etc.),which are information unavailable when optical transceivers weremanufactured. In another example, on near-end receive side, the settingof the DLI may shift 2˜3 mW over life or aging or other arbitrarycauses, which leads poor bit-error-rate (BER) if the wavelength of thesignal is held constant. A DLI control loop is able to ensure that theDLI is set at low signal-to-noise ratio state resulting in a convergedsmall BER. The DLI loop can be alternate executed with a BER-basedwavelength control loop. A spiral search approach is introduced forcapturing an off-center restart point of the DLI control of using twoDLIs for separately handling split TM mode and TE mode of a polarizedoptical signal. It is therefore advantageous for the closed looptechniques provided by the present invention to use the information andhence improved performance.

Embodiments of the present invention can be implemented in conjunctionwith existing systems and processes. For example, the back-channel datacan be implemented to be compatible with existing communicationprotocols and specifically be used for the near-end receiver tocommunicate with far-end transmitter for conducting the DLI control loopand the BER-based wavelength control loop in a coordinated manner.Back-channel data are used by optical transceivers that arepreconfigured to use them, and optical transceivers that are notconfigured to use the back-channel data may simply ignore them. Inaddition, optical transceivers according to embodiments of the presentinvention can be manufactured using existing manufacturing equipment andtechniques. In certain implementations, existing optical transceiverscan be upgraded (e.g., through firmware update) to take advantage of thepresent invention. There are other benefits as well.

The present invention achieves these benefits and others in the contextof known technology. However, a further understanding of the nature andadvantages of the present invention may be realized by reference to thelatter portions of the specification and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1 is a simplified diagram illustrating optical transceiveraccording to an embodiment of the present invention.

FIG. 2 is a simplified diagram illustrating an encoded data frameaccording to embodiments of the present invention.

FIG. 3 is a simplified diagram illustrating an optical transmitter withback-channel data control according to embodiments of the presentinvention.

FIG. 4 is a simplified diagram illustrating an optical receiveraccording to an embodiment of the present invention.

FIG. 5 is a simplified flow chart illustrating a method of optimizingcommunication signal in receive side according to an embodiment of thepresent invention.

FIG. 6 is a simplified flow chart illustrating a control method oftracking delay line interferometer (DLI) in receiver module according toan embodiment of the present invention.

FIG. 7 is a simplified flow chart illustrating a method of optimizingcommunication signal in transmit side in a coordinated control accordingto an embodiment of the present invention.

FIG. 8 is a simplified diagram illustrating a coordinated control fortracking optical signals alternately with a near-end DLI control andfar-end transmitter control in a communication system according to anembodiment of the present invention.

FIG. 9 a schematic diagram of spiral search of an off-center restartpoint in a two-dimensional parameter space according to a specificembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to communication systems and methods.According to an embodiment, delay line interferometers (DLIs) in areceiver module of an optical transceiver are configured with a controlloop to optimize overall bit-error-rate (BER) of communication signalsagainst any drift. The DLI control is further coordinated with activeBER-based wavelength control in a transmitter module of the opticaltransceiver, both being operated alternatively in time or frequency.There are other embodiments as well.

Most optical communication modules have some form of internal controlsystems to maintain the optical performance. For example, typicalcontrol parameters include optical power, wavelength, extinction ratio,and/or others. However, in most cases, conventional techniques for thetransmitting optical module to maintain these parameters rely on proxymeasurements. For example, transmitted optical power may be measured bya tap and photodiode, or extinction ratio may be inferred from amodulator bias. Unfortunately, these conventional techniques areinadequate. A difficulty is that these proxy measurements may notrepresent the actual transmission characteristics, and as a result thetransmitting optical path is not optimized.

In optical communication, another difficulty is that in an optical linesystem (including fiber optics, amplifiers,multiplexers/de-multiplexers, dispersion compensation, etc.), optimaltransmission parameters may not be constant and may in fact change dueto the line equipment or conditions. This may render the transmissionparameters even farther from optimal.

It is to be appreciated that embodiments of the present inventionprovide advantages over existing techniques. More specifically,embodiments of the present invention make use of digital signalprocessors (DSP) and forward error correction (FEC) modules on theoptical receive path. The inclusion of a DSP and FEC on the opticalreceive path within the module itself allows the receiving side todetermine the quality of the incoming optical signal. Additionally,embodiments of the present invention provide an advanced FEC encodingthat includes the ability to place additional digital informationalongside the transmitted data (“back-channel”), thereby allowing thereceive-side module to inform the transmitting-side module of thecurrent signal integrity.

With DSP/FEC and advanced FEC encoding working together, a closed-loopsystem can be implemented, where the optical parameters of the transmitside can be tuned to optimally to reflect the current opticalconditions. The tuning parameters include, but not limited to,compensating for aging or environmental effects of optical equipmentfrom the transmitting optical module through to the receiving opticalmodule.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

FIG. 1 is a simplified diagram illustrating optical transceiveraccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in FIG. 1,transceiver 100 includes an optical receiver 101 that interfaces with anoptical communication link and is configured to receive and processoptical communication signals. In various embodiments, optical receiverincludes various components, such as filter, transimpedance amplifier(TIA), fiber optic connectors, and others. Optical receiver 101 mayadditionally include optical transmission devices such as opticalamplifiers, optical attenuators, chromatic dispersion compensation(static or tunable), lengths of fiber, patch panels and patch cables,optical multiplexers, optical de-multiplexers, etc. Among otherfeatures, optical receiver 101 converts received optical signals toelectrical signals that can later be processed. The electrical signalsare then processed by various digital signal processors (DSP). Forexample, application specific integrated circuit (ASIC) 110 includes FECdecoder 111 and a back-channel detector 112.

It is to be appreciated that once back-channel data are detected fromthe incoming data stream, it is determined that the source of thereceived optical signals is compatible with the use of back-channel datafor adjusting its operating parameters. In various embodiments, the useof back-channel data is a part of a predetermined communication protocolthat two or more transceivers use. If back-channel data is not detectedfrom the received optical signals, the source of the received opticalsignals is not equipped to utilize back-channel data, and it would beunnecessary and even wasteful to perform signal measurements that are tobe embedded into back channel data.

ASIC 110 may also include a module for measuring and analyzing signalintegrity of the received signal (i.e., electrical signals convertedfrom the received optical signals). Signal integrity may be evaluated invarious signal measurements that include, but are not limited to overallsignal-to-noise ratio (SNR), individual PAM-4 level SNR, overall PAM-4histogram, optical eye diagram, and/or others. In additional to signalintegrity, data error rate associated with the incoming signal may beevaluated as well. For example, FEC decoder 111 determines error ratebefore performing error recovery. More specifically, FEC decoder 111 hasthe ability to calculate a bit error ratio (BER) prior to FEC errorrecovery. Depending on the implementation, BER can be calculated inseveral different ways, such as overall BER, individual lane BER,individual PAM-4 level BER (i.e., MSB BER, LSB BER), bit-transitionerror matrix (e.g., in PAM-4, BER for 0->1, 0->2, 0->3 and all otherlevel transitions), and/or other ways.

The back-channel detection module 112 is configured to detect whetherthe received signals include back-channel data that can be used tooptimize data transmission performance. For example, the back-channeldata are embedded by the source of the received signals (e.g., anotheroptical transceiver or communication apparatus). In various embodiments,the back-channel detection module 112 is coupled to a control module115. The control module 115 is configured to adjust various operatingand transmission parameters of transceiver 100 based on the back-channeldata. For example, operating parameters include temperature, biassettings, multiplexer settings, wavelength, and others, which aredescribed below. It is to be appreciated that the back-channel detectionmodule 112 may be implemented as a part of the closed feedback loop(e.g., between two optical transceivers). That is, data are transmittedto a second transceiver over an optical communication link. The secondtransceiver includes DSP and FEC module that measure the signal quality(e.g., SNR) and data quality (e.g., BER), and the measurement resultsare embedded in the back-channel data that are transmitted back totransceiver 100. The back-channel detection module 112 detects theexistence of the back-channel data, which are used by the control module115 to adjust operating parameters of transceiver 100. Depending on theoperating condition and specific implementation, there could beiterations of processes for changing parameters, receiving back-channeldata reflecting the signal quality associated with the changedparameters, and changing parameters again.

It is to be appreciated that, as explained below, back-channel data canbe used to adjust not only transmitter parameters for outgoing data, butalso receiver parameters for processing incoming data. For example, backchannel data can be used to adjust how incoming optical signals areprocessed.

In various embodiments, control module 115 stores near-end parameters,which may be determined at the time when the transceiver 100 ismanufactured. Control module 115 analyzes the received back-channeldata, which reflects the actual conditions of data transmission, and theadjustment of operating parameters can be modifying the existingparameter based on the existing near-end parameters. In variousembodiments, adjustment of operating parameters involves synchronizingand using both existing near-end data and the back-channel data thatreflects conditions for actual data communication.

Transceiver 100 includes an FEC encoder 114 and a back-channel insertionmodule 113 as shown. For example, the FEC encoder 114 and theback-channel insertion module 113 are implemented as a part of the ASIC110. It is to be understood that while FEC decoder 111 and FEC encoder114 are shown as two functional blocks in FIG. 1, FEC decoder 111 andFEC encoder 114 may be implemented a single FEC module. Similarly,back-channel detection module 112 and the back-channel insertion module113 can be implemented as single back-channel module.

FEC encoder 114 is configured to perform FEC encoding for electricalsignals that are to be transmitted through the optical transmitter 102.For example, FEC encoder 114 is configured to perform different types oferror correction. Back-channel insertion module 113 is configured toinsert back-channel data into the outgoing data stream that is to betransmitted. As explained above, back-channel data include informationregarding the quality of received data, which pertains to transmissionparameters and settings of the transmitting transceiver that sends datato transceiver 100. It is to be appreciated that the back-channelinsertion module is capable of inserting and/or detecting, with highfidelity, additional digital information alongside and withoutinterfering with the transmitted data. For example, a predefined segmentof outgoing data stream is used to embed the back-channel data.

In FIG. 1, a close loop technique is used for optical communication,with an optical transmitter and an optical receiver. It is to beunderstood that close loop techniques that use back-channel foroptimizing data communication can be used in other types ofcommunication links as well, such as existing communication lines withcopper wires and/or other mediums.

FIG. 2 is a simplified diagram illustrating an encoded data frameaccording to embodiments of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in FIG. 2, anexemplary FEC encoded frame can be divided into a most significant bits(MSB) section and a least significant bits (LSB) section. Both the MSBsection and the LSB section include their own respective headers. Forexample, the headers are 128 bits long. A header is then subdivided intoalignment marker region (0-63 bits), “mfas” region (64-71 bits),overhead region (72-120 bits), and “ecc” region (121-127 bits). It is tobe appreciated that overhead region stores back-channel data, whichincludes information related to quality (e.g., measured and/orcalculated) of received signals. For example, an optical transceiverthat is not equipped to take advantage of the back-channel data cansimply ignore and skip over the back-channel region.

Now referring back to FIG. 1. Outgoing electrical signals are convertedto optical signals and transmitted by the optical transmitter 102. Forexample, optical transmitter 102 includes one or more lasers devices(e.g., laser diode with cooling), one or more modulators. Additionally,optical transmitter 102 may include multiplexing and optical controlblocks. Implementation and operating parameters of optical transmitter102 usually have significant impact on signal quality and datatransmission performance of the outgoing data stream. By adjustingoperating parameters and settings of optical transmitter 102, signalquality and data transmission performance can be improved and optimized.While operating parameters and settings can be optimized initially atthe factory, being able to adjust these parameters and settings based onactual signal measurements is better, since actual signal measurementsreflect true operating conditions (e.g., fiber optic lines,interference, temperature, etc.).

According to various embodiments, the control module 115 of thetransceiver 100 processes the received back-channel data, which includeactual measurements of data quality as measured by a second transceiverthat receives data from transceiver 100. The control module 115 thendetermines the optical parameters and settings accordingly. For example,operating parameters and settings may include, but not limited to, thefollowing:

-   -   Laser temperature setting (or TEC current if directly        controlled)    -   Laser bias current    -   Modulator bias setting (e.g., heater power setting if a        thermo-optically controlled MZM)    -   Multiplexer offset bias setting (e.g., heater power setting if a        thermo-optically controlled DLI)

As an example, back-channel data provides signal quality informationthat can be used to adjust parameters of laser devices. Morespecifically, laser devices used for optical data transmission may becontrolled using temperature and bias control parameters. FIG. 3 is asimplified diagram illustrating an optical transmitter with back-channeldata control according to embodiments of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in FIG. 3, driver301 that generates driver signal based on outgoing data can be adjustedby a voltage swing parameter. Similarly, modulators 302 and 303 may beadjusted using settings such as RF amplitude, DC bias, and/or others.For example, modulators 302 and 303 may be implemented usingMach-Zehnder modulators (MZM). Light source for optical transmitter 300includes laser diodes 305 and 306. For example, laser diodes can beadjusted by changing laser bias and/or laser temperature. Similarly,delay line interferometer (DLI) 304, which functions as an opticalmultiplexer, can be adjusted with an offset bias. It is to beappreciated that the control module 320 of optical transmitter 300 canuse the back-channel data to determine which parameters (as listedabove) are to be adjusted. For example, the control module 320 has acontrol interface that provides control signals for the abovementionedparameters such as bias control, temperature control, swing voltage, andothers.

According to an embodiment, back-channel data are used as a part ofoptical transceiver. As an example, optical receiver 101 is a part ofthe transceiver 100 as shown in FIG. 1, and various operating parametersof optical receiver 101 may be adjusted based on back channel data. Inanother example, optical transmitter 102 is a part of the transceiver100 as shown in FIG. 1, and a transmitter setting of the opticaltransmitter 102 may be adjusted based alternative back-channel data. Acertain signal can be formulated and utilized in the back-channel datafor coordinating the controls in a near end optical receiver and a farend optical transmitter of a communication system.

FIG. 4 is a simplified diagram illustrating an optical receiver 400according to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. Optical receiver 400includes polarization beam splitter (PBS) 405 that splits the receivedoptical signal for processing. The received optical signal is thenprocessed by optical multiplexers 403 and 404. For example, multiplexers403 and 404 comprise DLI devices that can be adjusted using biassettings. For thermal-optically controlled DLIs, heater power settingsmay be used to adjust DLI operations. Multiplexers 403 and 404 arecoupled to photodetectors 401 and 402, which respectively convertincoming optical signals that demuxed by the DLI devices to currentsignals. The current gain settings of the photodetectors 401 and 402 canbe adjusted. For example, photodetectors may be amplified, and thus gainsettings are needed. For example, avalanche photodiodes can be adjustedby changing its photocurrent gain. The outputs of photodetectors 401 and402 are coupled to TIA 406, which generates voltage signals based on thecurrent signals converted from the received optical signals. Dependingon the implementation, various parameters such as amplitude, gain,and/or bandwidth, can be adjusted based on back-channel data. Asmentioned above, a control module 420 may be used to process receivedback-channel data and generates control signals to adjust theseparameters.

To make use of back-channel data and to generate control signals forchanging operating parameters, a control module can be used. Forexample, abovementioned control modules 420 may be implemented as a partof a computer engine block, or a microcomputer that is a part of opticaltransceiver ASIC. To use the transceiver 100 as an example, the controlmodule is configured with the back-channel insertion module 113 toinsert digital signals alongside the transmitted optical data, whichdescribe the integrity of the received optical signal (as measured byDSP and/or FEC modules). Additionally, the control module is able to usethe back-channel detector 112 to detect back-channel data embedded inthe received signals. Once detected, the control module processes theback-channel data and generates control signals accordingly. The controlsignals are used to adjust various operating parameters of thetransceiver (e.g., optical receiver, optical transmitter, etc.).Additionally, operating conditions of the optical transceiver may change(e.g., interference, optical line quality, temperature change, etc.). Byusing back-channel data, the control module adjusts and optimizestransceiver performance accordingly. Since the back-channel data areshared between two or more transceivers, two transceivers form afeedback loop for optimizing data transmission over a communicationlink.

In an exemplary embodiment, back-channel data are used to adjust, amongother parameters, transmitting wavelength. In a DWDM system, forexample, the transmitting wavelength is an important parameter. Incertain implementations, transmitting wavelength can be controlled viacarefully tuning the temperature of a thermally tuned laser. However,the actual frequency may not match the measured temperature of thelaser. More specifically, temperature and wavelength for the same laserdevice may change over time as a part of laser device aging process. Asa result, the thermal gradient of a laser device at the beginning oflife calibrations, typically due to aging, becomes inaccurate over anextended period of time. Furthermore, since laser devices generate heat,depending on the packaging and/or placement of the laser device,adjusting laser devices in actual operating condition. For example,thermal gradients are involved with either self-heating or environmentaltemperature interference.

In various embodiments, the control module is positioned on the datatransmission path. For example, the wavelength of the light is measuredusing Fabry-Pérot interferometer (or etalon) type of device. It is to beappreciated that even small changes in frequency and/or wavelength canhave significant impact on the signal integrity of the transmitted data.For example, in various embodiments of the present invention,characteristics of the received signal (e.g., SNR, BER, Eye-Levelparameters, etc.) are passed from a receiving optical transceiver backto the transmitting optical transceiver. The transmitting opticaltransceiver then adjusts the frequency of its one or more lasers andwaits for confirmation of whether an improvement has been made, or thesignal has gotten worse, thereby forming a closed feedback loop foroptimizing signal transmission. The transmitting optical transceiver cantry many frequencies (in the right direction) to obtain optimalfrequencies of the one or more lasers.

Below is a simplified process for adjusting optical transceiversaccording to embodiments of the present invention, the process includingthe following steps:

-   -   1. At the receiver, measure a bit-error rate BER (“b11”)        corresponding to conditions at transmitter temperature setting 1        (“t1”) and transmitter temperature setting 2 (“t2”), and the        receiver uses a predefined back-channel to transmit data back to        the transmitting module;    -   2. At the transmitting module, modify transmission parameter        with step t1 by an amount approximately equal to 0.5 GHz;    -   3. At the receiving module, measure BER (“b21”) and use the back        channel to transmit back to the transmitting module;    -   4. At the transmitting module, modify transmission parameter        with step t2 by an amount approximately equal to 0.5 GHz;    -   5. At the receiving module, measure BER (“b12”) and use the back        channel to transmit back to the transmitting module;    -   6. At the transmitting module, calculate values d1 and d2 (which        are changes to be made to temperature t1 and t2) with the        following equations (g is a gain setting for the transmitting        module):        d1=g×(1−b21/b11)  a.        d2=g×(1−b12/b11)  b.    -   7. At the transmitting module, adjust t1->t1+d1 and t2->t2+d2;        and    -   8. Repeat from step 1 as needed, until an acceptable BER is        obtained at the new t1 and t2 settings.

It is to be noted that steps 1-3 and 4-5 are performed in a changingorder to remove unrelated monotonic effects from the system.

It is to be appreciated the back-channel data can also be used toprovide modulator bias control. Among other things, the bias point ofthe modulator needs to be maintained at the proper value to provide anoptimized extinction ratio. In some cases, the optimal bias point is notat quadrature but rather at a point off-quadrature. Often, a fixed biaspoint is used, which based on for worst-case line system conditions. Invarious embodiments, characteristics of the received signal (includingSNR, BER, Eye-Level parameters, etc.) are measured by the receivingmodule, embedded into back-channel data, and to the transmitting module.The transmitting module adjusts the modulator bias point based on theback-channel data, and waits for indication (embedded in theback-channel data) from the receiving module as a feedback for the nextiteration of adjustment. In this way, the transmitting module cancontinuously seek the optimal modulator bias point until a predeterminedthreshold performance level is obtained.

The back-channel data in closed feedback loop can also be used foradjusting multiplexer bias settings. For example, in a silicon photonicsbased multi-wavelength module design, a delay line interferometer (DLI)may be used to multiplex two optical wavelengths onto the sametransmitting optical fiber. To optimize performance, center frequency ofthe DLI needs to be carefully controlled to optimally pass or separateboth wavelengths.

In an exemplary embodiment, the characteristics of the received signal(e.g., SNR, BER, Eye-Level parameters, etc.) from a transmitting moduleare measured by a receiving module, which inserts the signalcharacteristic information into back-channel data. The back-channel datais then transmitted to the transmitting module, along with other data.The back-channel data is then processed by the transmitting module. Thetransmitting module adjusts the DLI center frequency based on the signalcharacteristics provided in the back-channel data, and transmits data tothe receiving module with new DLI bias setting, and waits for signalcharacteristics information from the receiving module. The feedback loopbetween the transmitting module and the receiving module operates anumber of iterations until certain predetermined conditions are met. Forexample, predetermined conditions may include a predetermined number ofiterations, the total amount of adjustment/calibration time, and/orpredetermined signal characteristics.

In certain embodiments, near-end tuning (e.g., in combination with biassetting) of DLIs are adjusted using the closed-loop back channel data.

Additional parameters and settings of optical transceivers can beadjusted using back-channel data. For example, characteristics of thereceived signal (e.g., SNR, BER, Eye-Level parameters, etc.) aremeasured by a receiving module and inserted to the back-channel data asa part of the closed feedback loop. The transmitting module then usesthe back-channel data to adjust its operating parameters and settings,which include, but not limited, the following:

Modulator swing (RF amplitude);

Laser bias (i.e., Automatic Power Control);

PAM-4 level optimization (in this case the histogram calculated by thereceiver can be used to optimize the level amplitude settings);

PAM-4 MSB/LSB lane skew; and/or

Relative transmitted power among one or more lasers sharing the sameoptical fiber.

For example, by adjusting relative transmitted power among one or morelasers, optical SNR (OSNR) within a channel group can be adjusted tooptimize the overall BER. For example, in a 2-channel 100G system, thelaunch power and OSNR can be optimized for 100G BER and still keep thetotal transmitted optical power (CH1+CH2 power) constant.

In certain embodiments, in addition to using the back-channel totransmit data from the receiving module to the transmitting module, theSNR and BER can be used to optimize near-end parameters, which includesbut not limited to:

TIA output amplitude, gain or bandwidth control;

Photodiode gain (e.g., in an APD); and/or

Receiver de-multiplexer center frequency control (i.e., for a siliconphotonics DLI demultiplexer).

In various implementations, two or more parameters of an opticaltransceiver may be adjusted, and when doing so, priority or preferencemay be given to parameters such as far end TX wavelength tuning overnear end RX DLI tuning. For certain parameters, such as far endtransmission MSB/LSB and far PAM 4 swing settings, it may beadvantageous to perform optimization in parallel.

The optical receiver the communication system based on silicon photonicsalso uses a delay line interferometer (DLI) as a demuliplexer todemultiplex an incoming optical signal with two wavelengths from oneoptical fiber and split to two separate detectors. The set point of theDLI controls the splitting of the incoming optical signal. While invarious implementations, the DLI may be drifted over time so that itsset point could be far off center and even out of a threshold (region)to lock the DLI at a low signal-to-noise ratio state. Referring to FIG.4, the optical receiver 400, which may be disposed in a receive sideTransceiver of a communication system, includes a polarization beamsplitter (PBS) 405 to receive a polarized optical signal. PBS 405 splitsthe optical signal to a first part having a Transverse Magnetic (TM)mode received by a first DLI 403 in TM mode and a second part having aTransverse Electric (TE) mode received by a second DLI 404 in TE mode.Each of the DLIs 403 and 404 is used as a demultiplexer to split onebeam (either in TM or TE mode) to two beams respectively detected byphotodiodes PD1 401 and PD2 402 which work together with a TIA module406 to convert the optical signals to electrical current signals.Through the above optical path for the TM mode and TE mode, a set pointof the DLI that controls a heating power of a heater associated with theDLI needs to be adjusted properly to achieve a desired phase delay tocause a demultiplexing of the polarized optical signal with highextinction ratio. In case there is a shift of the set point of at leastone of the two DLIs, the phase delay is changed accordingly which causesfalse mixing of different wavelength or polarization mode and introducesnoises or errors into the electrical signals generated by the TIAmodule.

In an embodiment, a receiver DLI (Rx DLI) control loop structure isproposed, as shown in FIG. 4, to use a control module 420 to couple withthe one or more DLIs 403 and 404 and a method of executing the Rx DLIcontrol loop is implemented for tracking the incoming optical signalsfrom the receive side of the communication system. FIG. 5 is a flowchart illustrating a method of optimizing communication signal inreceive side according to an embodiment of the present invention. Thisdiagram is merely an example, which should not unduly limit the scope ofthe claims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in FIG. 5, themethod is initiated with a step of checking if the DLIs associated withthe optical receiver is locked in a low signal-to-noise ratio (SNR)state. In this step, the control module 420 is configured to calculate afirst bit-error-rate (BER) of an incoming electrical signal convertedfrom an incoming optical signal received by the DLIs before beingdecoded by a FEC module. The control module 420 is also configured tocompare the first BER to a threshold. For example, a threshold BER of1e-3 is selected. If the first BER is smaller than the threshold, theDLIs is considered to be locked in the low SNR state, but furtheroptimization of the BER is needed for overcoming possible set pointshift over time or aging or any other reasons.

In an embodiment, after determining the DLIs to be a low SNR state, themethod includes a step of starting a first iterated operation executedin a Rx DLI control loop for optimizing a set point of each DLI in thenear end receiver using the BER as a control parameter. In a specificembodiment, the set point of a DLI is a bias point for controllingheating power for operating a heater associated with at least one arm ofthe DLI for tuning its phase delay by changing temperature to achievedesired interference spectrum for demuxing the optical signal. Referringto FIG. 5, the iterated operation is performed in the Rx DLI controlloop to adjust the set point in multiple iterations until it issatisfied under a convergence criterion. For example, the convergencecriterion can be satisfied with a small resulting change of the setpoint of the DLI, which indicates the set point is substantiallyoptimized to yield a second BER that is desirably smaller than the firstBER and substantially converged to keep the DLI at the low SNR state.

In another embodiment, the Rx DLI control loop can be used to catch anoff-center restart point. Referring to FIG. 5, in case the first BER isdetermined in the first step to be greater than the threshold, the RxDLI is not locked in a low SNR state. This occurs at least in twoscenarios: 1) the set point of Rx DLI has drifted too far from a nominalset point or 2) the wavelength of the optical signal has drifted toomuch away from a target wavelength set for a far-end transmitter in thecommunication system where the incoming optical signal is generated. Forthe scenario 1) with off-centered DLI setting, the method proposed inthe flow chart of FIG. 5 includes searching of the Rx DLI parameterspace to catch an off-center restart set point. Optionally, the Rx DLIincludes one TM_DLI for handing TM-mode signal and one TE_DLI forhandling TE-mode signal. The set point includes a first bias point forcontrolling the TM_DLI and a second bias point for controlling theTE_DLI, both contributing the overall BER finally after the TM-mode andTE mode optical signal are combined and converted to an electricalsignal. Then, the searching of the Rx DLI parameter space is atwo-dimensional search with respective up-down movement in twodirections of the first bias point and the second bias point. Of course,if the set point involves more parameters, the searching includes allcombined up-down movements of all parameters.

FIG. 9 shows an example of a two-dimensional search for finding anoff-center restart point in a type of spiral search. The circled regionto the right side of the figure represents a contour map of the BERvalues in the plane of two bias points for TM-mode and TE-mode control,within which the BER values are smaller than a threshold, i.e., lockedregion. The darker in the circled region is toward a central region1000, the smaller the BER value is. All plane area outside the circledregion, e.g., region 800, represents an un-locked region with the BERvalues greater than the threshold. Since the set point has drifted toofar, the beginning point 500 is found at outside the circled region.Starting from the beginning point 500 (or firstly shifted to alast-known good point), the spiral search includes multiple searchsub-steps to make a serial movements in both a TM direction and a TEdirection in a spiral pattern. The search loop continues each searchsub-step followed by a lock-detection step up to end-of-life limits andmay eventually end with finding a restart point 900 inside the circledregion. The restart point 900 may still be an off-center point but canbe used to start the first iterated operation in the Rx DLI control loop(FIG. 5) and drive the set point towards the central point 1000 as thecontrol parameter BER converges.

For the scenario 2) with off-target wavelength drift in the opticalsignal originally generated from a far end transmitter of thecommunication system, Referring to FIGS. 1-4, the Rx DLI control loop inthe near end receiver 400 can communicate with the far end transmitter300 by taking advantage of the back channel data insertion and detectionfunction in the transceivers 100 at both the near end and the far end ofthe communication system. Optionally, at the beginning of executing thefirst iterated operation of Rx DLI control loop, the control module ofthe near end receiver is configured to send a constant BER on the backchannel. The transmitter 300 at the far end is able to detect theconstant BER in the back-channel data and use it as a control signal fora control module 320 to stop any wavelength adjustment so that theoptical signal generated by the far end transmitter is held at a stablecondition as the near end receiver is tracking the Rx DLI set point.Optionally, at the end of the iterated operation as the second BERconverges the control module of the near end receiver is also configuredto hold the set point of DLI at the last adjusted or optimized constantto keep the near end receiver in working and send a real-time BER on theback channel. The far end transmitter 300 is also able to detect thereal-time BER in the back-channel data and use it as a control signalfor the control module 320 to initiate a second iterated operation fortracking the wavelength of the optical signal. Since the optical signalgenerated by the far end transmitter 300 is actually the incomingoptical signal received by the near end receiver 400, the seconditerated operation optionally still uses the BER calculated by thecontrol module 420 at the near end receiver 400 as a control parameterto determine if the second iterated operation is converged based ondetection that the BER is converged to a third BER that is no greaterthan the second BER. Referring to FIG. 5, the second iterated operationis coordinately performed with the first iterated operation throughback-channel data communication.

It is to be appreciated that different types of algorithms may beimplemented to take advantage of the feedback mechanisms involvingback-channel data. For example, a transmitting transceiver keepsadjusting its operating parameters based on the measured signalcharacteristics provided by the receiving transceiver until performancelevel in terms of a bit-error-rate threshold is obtained. In certainimplementation, optical transceivers perform quality continuously and aslong as they operate. There are other implementations as well.

In a specific embodiment, the first iterated operation of the Rx DLIcontrol loop is executed in a gradient descent search of an optimizedset point of the DLI using the BER as a proportional integral derivativeparameter. FIG. 6 is a simplified flow chart illustrating a controlmethod of tracking the set point of DLI in the near end receiveraccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown in FIG. 6, aniteration of the loop employs a dithering and adjusting process toadjust DLI bias point based on the BER as it responds to both up anddown dithers of the bias point. Assuming that the near end receiver (seeFIG. 4) has a TM_DLI for demuxing the TM-mode signal and a TE_DLI fordemuxing the TE-mode signal of a same incoming optical signal, thedithering and adjusting process includes operations for dithering andadjusting two independent bias points: a TM bias point and a TE biaspoint.

Referring to FIG. 6, the iteration includes (1) adjust the TM bias pointup from a nominal set point in one dither step, for example, the TM biaspoint is adjusted to change heating power of 0.25 mW to achieve a phasechange of 3 degrees up for the DLI; (2) measure the BER at thiscurrently dithered point to obtain a BER(TM+) value; (3) adjust the TMbias point down from the nominal set point in another dither step, forexample, the TM bias point is adjusted to change heating power of 0.25mW to achieve a phase change of −3 degrees down for the DLI; (4) measurethe BER at the current dithered point to obtain a BER(TM−) value;(5)-(8) repeat the steps of (1)-(4) for dithering the TE bias point andmeasuring corresponding BER values, BER(TE+) and BER(TE−); (9) adjustthe TM nominal set point by adding TM_gain×[1−BER(TM+)/BER(TM−)]; (10)adjust the TE nominal set point by adding TE_gain×[1−BER(TE+)/BER(TE−)];and measuring the overall BER at the adjusted bias points, based onwhich a determination on whether the Rx DLI control loop is converged ornot is checked. Note, the sequential number used here does not limit anactual implementation order of each dither step. For example, steps (3)and (4) may be performed before steps (1) and (2), steps (5)-(8) may beperformed before steps (1)-(4). In another example, steps (1)-(2) andsteps (3)-(4) are performed in a changing order and steps to removeunrelated monotonic effects from the system. Similarly, steps (5)-(6)and steps (7)-(8) are performed in a changing order and steps to removeunrelated monotonic effects from the system. It is to be appreciatedthat dithering and adjusting for TM_DLI and dithering and adjusting forTE_DLI can be alternately performed to obtain an overall BER in everyiteration of the first iterated operation towards convergence, althoughother variations are still possible.

In some embodiments, the first iterated operation is performed until afirst convergence criterion is reached. Optionally, the firstconvergence criterion is defined by a limited recent average movement ofa set point of the DLI in recent iterations of the first iteratedoperation. In other words, referring to FIG. 9, the set point issubstantially close to the target point 1000 such that the BER value forany dither-up step and dither-down step is substantially the same.Optionally, the first convergence criterion can be defined as a limitedchange of the BER over recent iterations of the first iteratedoperation. In other words, a difference of the BERs obtained between twoconsecutive iterations of recent iterations is substantially the same.Optionally, the first convergence criterion can be defined as a fixednumber of dither steps for increasing or decreasing delay-phases of theDLI around a nominal set point over recent iterations of the firstiterated operation. The fixed number is merely a predetermined number ofiterations expecting that an exponential convergence to target point canbe reached under the proportional integral derivative loop regardless ofhow far off the start point was.

In another specific embodiment, the second iterated operation isexecuted in a gradient descent search of an optimized setting of the farend transmitter using the BER measured at the near end receiver as aproportional integral derivative parameter. FIG. 7 is a flow chartillustrating a method of optimizing communication signal in transmitside in a coordinated control according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. As shown inFIG. 7, the method of optimizing communication signal in transmit sideonly starts once the BER is optimized in receive side (see FIG. 5). If aconstant BER is detected in back-channel data by the control module 320of the far end transmitter 300 (see FIG. 3), the control module 320 onlyexecute dithering a transmitter setting without adjusting transmissionfrequency in the transmitter wavelength control loop. The seconditerated operation of optimizing communication signal is actuallystarted when a constant BER is not detected but a real-time BER isdetected in the back-channel data indicating the active BER has beenoptimized as the Rx DLI control loop ended with a converged second BER(assuming that the Rx DLI control is started with a first BER). Thistime, the control module 320 executes both dithering and adjusting thetransmission frequency in each dither step and adjust nominaltransmitter setting to adjust wavelength of the optical signal. Thesecond iterated operation still use the BER at the receive side as acontrol parameter to determine if the BER is converged (see FIG. 5) at athird BER having a value no greater than the second BER based on asecond convergence criterion.

In some embodiments, the second iterated operation is performed untilthe second convergence criterion is reached. Optionally, the secondconvergence criterion is defined by a limited change of the BER overrecent iterations of the second iterated operation. In other words, adifference of the BERs obtained between two consecutive iterations ofover recent iterations is substantially the same. Optionally, the secondconvergence criterion comprises a fixed number of dither steps forincreasing or decreasing transmission frequency around a nominal setpoint of the far end transmitter over recent iterations of the seconditerated operation. The fixed number is merely a predetermined number ofiterations expecting that an exponential convergence to target point canbe reached under the proportional integral derivative loop regardless ofhow far off the start point was. Optionally, the second convergencecriterion comprises a limited recent average movement of a set point ofthe far end transmitter in recent iterations of the second iteratedoperation. In other words, referring to FIG. 9, the set point issubstantially close to the target point 1000 such that the BER value forany dither-up step and dither-down step is substantially the same.

In another aspect, the present invention provides a communication systemwith signal tracking. The system includes a communication link, a firsttransceiver comprising a first control module and a first receiver, anda second transceiver comprising a second control module and a secondtransmitter. The second transceiver is disposed at far end of thecommunication system configured to send an optical signal to the firsttransceiver disposed at near end of the communication system. The secondtransceiver is also configured to receive back-channel data from thefirst transceiver. In some embodiments, the first receiver is configuredto receive the optical signal generated by the second transmitter andconvert the optical signal to an electrical signal. In some embodiments,the first control module is configured to calculate a bit-error-rate(BER) based on the electrical signal and to execute a first iteratedoperation of optimizing the first receiver until the BER converges. Insome embodiments, the second transmitter is configured to transmit theoptical signal to the first receiver with wavelength locking. In someembodiments, the second control module is configured to execute a seconditerated operation of optimizing wavelength of the optical signal. Thesecond iterated operation is alternate in time with the first iteratedoperation coordinately controlled by a back-channel data transmittedfrom the first transceiver to the second transceiver.

FIG. 8 is a simplified diagram illustrating a coordinated control fortracking optical signals alternately with a near-end DLI control andfar-end transmitter control in a communication system according to anembodiment of the present invention. This diagram is merely an example,which should not unduly limit the scope of the claims. One of ordinaryskill in the art would recognize many variations, alternatives, andmodifications. As shown in FIG. 8, in a communication system including areceive side transceiver and a transmit side transceiver communicated toeach other via a communication link, both Rx DLI control at receive sideand active BER-based wavelength control at transmit side are needed fortracking optical signals sent from a far end transmitter to a near endreceiver. Optionally, the tracking optical signal at the receive sidecan be operated in a coordinated manner by alternately performing areceive side Rx DLI control and a transmit side wavelength controlcontrolled by a back channel data inserted by the near end receiver anddetected by the far end transmitter.

Referring to FIG. 8, as an example, the receive side Rx DLI controlincludes alternate first stage of dithering and adjusting both setpoints for TM_DLI and TE_DLI and second stage of keeping set pointsconstant. Accordingly, a constant BER is transmitted in back-channelduring the first stage while a real-time BER is transmitted during thesecond stage. Again in this example, the transmit side wavelengthcontrol includes alternate third stage of dithering transmissionfrequency only and fourth stage of dithering and adjusting transmissionfrequency. In a coordinated manner, the third stage of the transmit sidewavelength control is executed whenever a constant BER is detected inthe back-channel data, which corresponds to the first stage of thereceive side Rx DLI control. The fourth stage of the transmit sidewavelength control is executed whenever a real-time BER is detected inthe back-channel data, which corresponds to the second stage of thereceive side Rx DLI control.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A communication system with signal tracking, thecommunication system comprising: a communication link; a firsttransceiver comprising a first control module and a first receiver; asecond transceiver comprising a second control module and a secondtransmitter, the second transceiver being configured, via thecommunication link, to send an optical signal to the first transceiverand receive back-channel data from the first transceiver; wherein: thefirst receiver is configured to receive the optical signal and convertthe optical signal to an electrical signal; the first control module isconfigured to calculate a bit-error-rate (BER) based on the electricalsignal, to execute a first iterated operation of optimizing the firstreceiver; the second transmitter is configured to generate the opticalsignal and transmit to the first receiver; the second control module isconfigured to execute a second iterated operation of optimizingwavelength of the optical signal, the second iterated operation beingalternate in time with the first iterated operation controlled by aback-channel data transmitted from the first transceiver to the secondtransceiver; the first control module is configured to couple with atleast a delay line interferometer (DLI) in the first receiver and toexecute the first iterated operation using the BER as a controlparameter in response of dithering and adjusting a set point of the DLI;wherein the communication link comprises an optical fiber.
 2. Thecommunication system of claim 1 wherein the first control module isconfigured to execute a search of a restart point in a two-dimensionalregion defined by a first bias point for the DLI.
 3. The communicationsystem of claim 1 wherein the second control module is configured tocouple with the second transmitter and to execute the second iteratedoperation using the BER as a control parameter in response of ditheringand adjusting a transmission frequency, wherein the first iteratedoperation and the second iterated operation are executed alternate intime.
 4. The communication system of claim 1 wherein the first controlmodule is configured to transmit a constant BER in the back-channel dataduring the first iterated operation until a first convergence criterionfor the DLI is satisfied, and to hold a set point of the DLI that isadjusted in a last iteration of the first iterated operation andtransmit a real-time variable BER in the back-channel data once thefirst convergence criterion is satisfied.
 5. The communication system ofclaim 1 the second control module is configured to start the seconditerated operation by dithering and adjusting the transmission frequencyupon a reception of the real-time variable BER in the back-channel datauntil a second convergence criterion for the second transmitter issatisfied, and to stop adjusting the transmission frequency despitedithering upon a reception of the constant BER in the back-channel data.6. The communication system of claim 4, wherein the first convergencecriterion comprises a limited recent average movement of a set point ofthe DLI in recent iterations of the first iterated operation.
 7. Thecommunication system of claim 4, wherein the first convergence criterioncomprises a limited change of the BER over recent iterations of thefirst iterated operation.
 8. The communication system of claim 4,wherein the first convergence criterion comprises a fixed number ofdither steps for increasing or decreasing delay-phases of the DLI arounda nominal set point over recent iterations of the first iteratedoperation.
 9. The communication system of claim 5, wherein the secondconvergence criterion comprises a limited change of the BER over recentiterations of the second iterated operation.
 10. The communicationsystem of claim 5, wherein the second convergence criterion comprises afixed number of dither steps for increasing or decreasing transmissionfrequency around a nominal set point of the second transmitter overrecent iterations of the second iterated operation.
 11. Thecommunication system of claim 5, wherein the second convergencecriterion comprises a limited recent average movement of a set point ofthe second transmitter in recent iterations of the first iteratedoperation.
 12. A method for tracking an optical signal in receive sideof a communication system, the communication system comprising: acommunication link; a first transceiver comprising a first controlmodule and a first receiver; and a second transceiver comprising asecond control module and a second transmitter, the second transceiverbeing configured to send an optical signal via the communication link tothe first transceiver and receive back-channel data from the firsttransceiver; the method comprising: receiving an optical signal by thefirst control module coupled with a delay line interferometer (DLI) inthe first receiver; calculating a first bit-error-rate (BER) of theoptical signal; determining the DLI to have a low signal-to-noise ratiobased on that the first BER meets a threshold; configuring a constantBER to a back-channel data; transmitting the back-channel data from thefirst receiver to a second transmitter to generates an optical signalwith a fixed wavelength; adjusting a set point of the DLI in a firstiterated operation for the optical signal transmitted from the secondtransmitter to the first receiver to result in a second BER of theoptical signal converged to a value smaller than the first BER.
 13. Themethod of claim 12 wherein the communication link comprises an opticalfiber.
 14. The method of claim 12 wherein the first iterated operationcomprises dithering and adjusting the set point of the DLI using thefirst BER as a proportional integral derivative parameter in a gradientdescent search of an optimized set point until the second BER satisfiesa first convergence criterion.
 15. The method of claim 14 wherein thefirst convergence criterion comprises a limited recent average movementof a set point of the DLI in recent iterations of the first iteratedoperation.
 16. The method of claim 14 further comprising holding theoptimized set point of the DLI and transmitting the second BER in realtime as back-channel data from the first receiver to the secondtransmitter of the communication system to adjust a transmitter settingin a second iterated operation to result in a third BER converged to avalue no greater than the second BER.
 17. The method of claim 16 whereinthe second iterated operation comprises dithering and adjusting thetransmitter setting using the second BER as a proportional integralderivative parameter in a gradient descent search of an optimizedtransmitter setting to obtain a third BER until a second convergencecriterion is satisfied.
 18. The method of claim 17 wherein the secondconvergence criterion comprises a limited change of the third BER overrecent iterations of the second iterated operation.
 19. The method ofclaim 17 further comprising, based on a determination that the first BERdoes not meet the threshold, searching a parameter space associated withthe set point of the DLI from a current set point or a last-known goodset point to search a restart set point associated with a fourth BERuntil the fourth BER meets the threshold.
 20. The method of claim 19wherein searching a parameter space comprises performing a spiral searchin a two-dimensional parameter space defined by a first set point forcontrolling a first DLI receiving a Transverse Magnetic mode signal ofthe optical signal and a second set point for controlling a second DLIreceiving a Transverse Electric mode signal of the optical signal.